Electronic spectra of the linear cationic chains NC2nN+ (n=1-7): an ab initio study

The ground-state equilibrium geometries of the linear carbon chain cations NC2nN+ (n = 1-7) have been investigated with B3LYP, CAM-B3LYP, and RCCSD(T) calculations. The ground state (X-2 Pi(g/u)) and excited state (1(2)Pi(u/g)) have been optimized by using the complete active space self-consistent field method. The present study reveals that these linear cations generally have the characteristic of bond length alternation in both electronic states. The vertical excited energies for the dipole-allowed (1, 2, 3)(2)Pi(u/g) <- X-2 Pi(g/u) transitions as well as the dipoleforbidden 1(2)Phi(u/g) <- X-2 Pi(g/u) transitions have been computed with the complete active space second-order perturbation theory. The calculated transition energies of 1(2)Pi(u/g) <- X-2 Pi(g/u) for NC2nN+ (n = 1-6) in the gas phase are 2.26, 2.09, 1.91, 1.72, 1.56, and 1.39 eV, respectively, which mutually agree well with the available experimental values of 2.11, 2.07, 1.88, 1.67, 1.49, and 1.34 eV. Moreover, the corresponding absorption wavelengths are predicted to have the significant nonlinear size dependence, which is different from the bands origin in NC2nN (n = 1-7).